The mounting density of electronic devices on a circuit board in a circuit board unit comprised of a circuit board on which a plurality of electronic devices are mounted has been rising in recent years. On the other hand, the electronic devices have also been evolving so as to be compatible with the higher densities. One way in which they have evolved is the broader use of ball grid arrays (BGAs), chip size packages (CSPs), and other large sized electronic devices. These large sized electronic devices are multi-pin devices. Most are structures with a large number of groups of terminals on the bottom surfaces (base surfaces) of the devices. These are effective for saving space, so they are starting to be used in large numbers in mobile phones, notebook PCs, etc.
However, BGAs and other large sized electronic devices are extremely expensive since they contain CPUs and other circuit modules in a high density. Therefore, when for example a problem is discovered at the connection portion between a large sized electronic device and the circuit board in the mounting process of the circuit board unit, the practice has been to not scrap the circuit board unit, but to detach the device in question once from the circuit board, make predetermined repairs, then reconnect it by soldering.
Detachment of an electronic device from a circuit board in this way is generally called “repair”. If the electronic device to be detached is a large sized electronic device (BGA etc.), since that device is large in size and has multiple pins, its heat capacity is large and repair is therefore not so easy. Further, in recent years, the general practice has been to use lead-free solder as the above solder. Compared with the conventional lead-based solder, repair is no longer so easy.
In this way, with repair becoming more difficult than the past, the general practice has been to perform repairs using a rework machine designed exclusively for repair. This is called a “repair system”.
Note that as known art relating to the present invention explained below, there are Japanese Patent Publication No. 11-135895, Japanese Patent No. 3470953, and Japanese Patent Laid-open Publication No. 9-283915.
A conventional repair system will be explained in detail with reference to FIG. 15. This is configured by a preheating device for preheating a soldered joint of an electronic device to be repaired mounted on a first surface (front) of the circuit board (hereinafter also referred to as a “repair device”) by a bottom heater from a second surface (back) of the circuit board and a main heating device for heating the preheated repair device concentrately by a top heater from the first surface of the circuit board. By the main heating by the top heater, the solder at the soldered joint of the repair device melts and the repair device can be taken off the circuit board.
In such a conventional repair system, as explained by the later explained FIG. 15, there are two problems. The first problem is the problem that the quality of the devices mounted on the surface (second surface) at the opposite side to the surface where the repair device is mounted ends up being degraded by the high temperature heating by the top heater.
The second problem is the problem that due to the high temperature heating by the bottom heater, the circuit board (for example, made of glass epoxy) reaches the glass transition temperature (Tg) and the circuit board as a whole ends up “warping”. This lowers the reliability of the soldered joints.